They changed the process for these fets, they are not low noise like they
used to be 40 years ago. To quote John Curl: "I found that Siliconix J110's , that I originally spec'd for the JC-2, over 35 years ago,
got VERY noisy in the late 70's, and remain so today, because they switched to ion implantation."

then the right question should be: which is the most similar device respect to the original one, 212/75? Considering a 170 with the same Idss of a 246, from datasheet the first has a gm of 22 and the second of 9. The J212 datasheet (at least the one that is available now) says a gm of 12.
Where am I wrong (and sure I am )?

Quote:

Originally Posted by AndrewT

the 246/103 is not a good replacement for the 170/74
I think the specified grade for the 170/74 is BL. GR is too low Idss and that limits the gain and transconductance.

I just had a look at FET Audio | Hi-End Audio Projects (Spencer is a member here),
an they still have 2SJ74 / 2SK170 matched in BL grade.
Spencer even has matched quads (quote: "Maximum match is 4 pcs K170 and 4 pcs J74 with same idss match together in one group.")
Matched quads is what I used with good results for my build.

...and then another question arise: if I don't use the balance switch, should I use the resistor combination of the "zero" position?
...

If you combine all resistors in the feedback loop (at the "zero" position) you
get 21.3 kOhms. With the 2kOhms to ground this is 21 dB gain.
If this suits your needs depends on the voltage gain of your power amplifier.
Personally I have never needed to use the "high" position on my ML-1 ("low"
attenuates by approx. -10dB this results in a total gain of 11dB).

I don't need a lot of gain, I think that 10dB is plenty.
I am wondering now if it is better to use the "low" gain setting (attenuating the input) and a "zero position setting" (or "higher feedback setting") of the feedback or a zero attenuation and a "higher feedback setting"......

Am I worring too much?

Thank you Georg

Quote:

Originally Posted by gk7

If you combine all resistors in the feedback loop (at the "zero" position) you
get 21.3 kOhms. With the 2kOhms to ground this is 21 dB gain.
If this suits your needs depends on the voltage gain of your power amplifier.
Personally I have never needed to use the "high" position on my ML-1 ("low"
attenuates by approx. -10dB this results in a total gain of 11dB).

5.6mA is the highest Idss for a quad match I can reach. Is it too low or it is worth trying?

5.6mA seems rather low to me, this is 1/3 of the "V" parts I use. I would not
cheap out on these parts and order them form Spencer ("BL" grade), as long as
he still has 2SJ74 available, and ask for the highest Idss matched quads he
can offer (should be around 10mA or so).

Quote:

Originally Posted by pidigi

I don't need a lot of gain, I think that 10dB is plenty.
I am wondering now if it is better to use the "low" gain setting (attenuating the input) and a "zero position setting" (or "higher feedback setting") of the feedback or a zero attenuation and a "higher feedback setting"...

I can confirm that the "-4dB" setting (this is 17dB gain from the line stage) is
stable; how much feedback you can apply to lower the gain without stability problems
I donīt know. (I would estimate that the gain has to be > 3 at least).